Furnace for production of controlled furnace atmosphere with recuperative preheating



July 16, 1957 F. A. RUSCIANO 2,799,491

FURNACE FOR PRODUCTION OF CONTROLLED FURNACE ATMOSPHERE WITHRECUPERATIVE PREHEATING Filed Dec. 17, 1954 5 Sheets-Sheet l INVENTOR. FA RUSCIANO /1/l www AT TORNEY 2,799,491 CE ATMOSPHERE 5 summer-smeet swww@ AT ORNEY F. A. RUSCIANO July 16, 1957 FURNACE FOR PRODUCTION 0FCONTROLLED FURNA WITH'RECUPERATIVE PREHEATING Filed Dec. 17, 1954 F. A.RusclANo 2,799,491

WITH RECUPERATIVE: PREREATTNG 5 Sheets-Sheet 4 July 16, 1957 FURNACE FORPRODUCTION 0F CONTROLLED FURNACE ATMOSPHERE Filed Deo. 17, 1954 W4,M4/,e4

AT ORNEY July 16, 1957 F. A. RusclANo 2,799,491

, FURNACE FOR PRODUCTION 0F CONTROLLED FURNACE ATMOSPHERE WITHRECUPERATIVE PREHEATING Filed Dec. 17, 1954 5 Sheets-Sheet 5 //j/A/H iINVENTOR. F.A.RU$CIANO ATTORNEY United States Patent O FURNACE FORPRODUCTION F CONTROLLEH) FURNACE ATMOSPHERE WITH RECUPERA- TIVEPREHEATENG Frank A. Rusciano, New York, N. Y., assigner to MetailurgicalProcesses Co., Newark, N. J., a corporation of New Jersey ApplicationDecember 17, 1954, Serial No. 475,953

11 Claims. (Cl. 266-5) This invention relates to a method for producingrapid and efficient heating of metal to high temperatures undernon-scaling conditions and to a furnace structure by which such methodmay be practiced.

In an application of F. A. Rusciano et al., Serial No. 347,716, ledApril 9, 1953 and entitled Method and Apparatus for Producing ControlledFurnace Atmospheres a method is disclosed for producing a non-scalingatmosphere to which the work is subjected during heating and which iscomposed of the direct products of combustion of the furnace burners.Briefly, this method comprises adjusting the air-fuel ratio of theburners so that the reaction products, when the reactions are carried tocompletion, will have a CO2/CO and H2O/H2 ratio of such value that thesum thereof is equal to unity. The reactions resulting from such amixture, while producing an atmosphere which is completely non-oxidizingto steel, release a relatively small percentage of the available B. t.u. of the fuel, of the order of 20%V to 25%, and therefore, produce alow heating rate and a restricted upper temperature limit. In theprocess of the aforesaid application, these limitations are overcome bythe subsequent combustion of this protective atmosphere gas externallyof the work chamber and out of contact with the work but in heattransfer relation thereto.

The present invention utilizes the above mentioned feature of theforegoing application, one of its object being to increase the operatingefciency of the process.

A further object is to provide a novel furnace structure for effectingthe secondary combustion of the protective atmosphere gases and formaking available to the work a greater proportion of the heat generatedby said secondary combustion.

Other objects and advantages will hereinafter appear.

In accordance with the present invention the heating of the metal isconducted in two stages, namely a preheating stage and a final heatingstage. The preheat stage may increase the metal temperature up to from1000 F. to l500 F. whereas in the final heating the metal may attain atemperature as high as 2500 F. In the preferred form of the inventionboth the preheating and the nal heating are carried out in the richcombustible protective atmosphere although, as Will be described, theinitial heating, up to temperatures of the order of 1200 F., may beeffected in a normal combustion atmosphere since no appreciable scalingoccurs up to such temperatures, particularly when employing a rapidheating rate. Moreover any slight oxidation which may occur in thepreheating step may be reduced in the subsequent high temperatureheating in the non-oxidizing atmosphere. In carrying on the presentprocess both the primary and secondary combustion are completed in heattransfer relation to the metal in its final heating stage. Thereafter,the products of the secondary combustion, which are at a temperaturesubstantially equal to or higher than the final temperature of the2,799,491 Patented July 16, 1957 ICC metal being heated, are utilized toheat the work in the preheat stage. Since the temperature of the metalbeing heated in this stage may range from room temperature up to 1200 F.or possibly 1500 F., a large proportion of the residual heat of thesecombustion products will be transferred to the work. The process isapplicable either to continuous or batch type furnaces.

In one form of the furnace illustrated, it comprises a linear orstraight through heating chamber in which the work passes from one endto the other. This chamber is divided into two heating zones, the firstextending from the charging opening to a point at which the work willhave attained in the normal travel period, the desired preheattemperature. While 1200 F. to 1500 F. has been mentioned, it is to beunderstood that these temperatures are only by way of example, theselected preheating temperature normally being dependent upon the mosteconomical length of the preheat zone. The final heating zone extendsfrom the end of the preheating zone to the discharge end of the furnace.This second zone is provided with a hot atmosphere produced by thereaction of combustion of a fuel and air mixture having a suiiicientdeficiency of air required for complete combustion, of the order of 50%,so as to render the atmosphere non-scaling to the work at the elevatedtemperature attained in the zone. This non-scaling atmosphere isgenerated by burners directly associated with this zone. The preheatzone is in open communication with the iinal heat zone and will receivesome of the atmosphere generated therein. However, the ventingarrangement of the furnace is designed to supply the major portion ofthis atmosphere directly to a second combustion chamber disposeddirectly over the final heating zone and separated therefrom by arelatively thin partition or arch of good heat conducting material. Thisatmosphere is highly combustible, still containing about to 80% of theB. t. u. content of the original fuel. This potential heat is convertedinto sensible heat by completing the combustion thereof with additionalair in the second combustion chamber whereby the additional heat thusgenerated may be utilized to increase the temperature and heating rateof the iinal heating zone. The products of this secondary combustion arethereafter passed in heat transfer relation to the work in thepreheating zone.

The invention will be more fully understood by reference to thelaccompanying drawings in which:

'Fig 1 is a central vertical longitudinal section of a continuousfurnace embodying the features of the invention;

Fig. 2 is a vertical transverse sectional view of the furnace, taken onthe line 2--2 of Fig. 1;

'Fig 3 is a vertical transverse sectional View, taken on the line 3-3 ofFig. l;

Fig. 4 is a central vertical longitudinal sectional view of a modifiedform of furnace;

Fig. 5 is a vertical transverse sectional View, taken on the line 5 5 ofFig. 4;

Fig. 6 is a central vertical longitudinal sectional view of a batch typefurnace embodying the present invention; and

v Fig. 7 is a transverse sectional view taken on the line 7--7 of Fig.6.

Reference will first be made to Figs. l, 2 and 3 in which a continuousfurnace of the gravity or rolldown conveyor type is shown. This furnaceis composed of refractory lbrickworlr and includes an inclined floor 10,opposite side walls 11 and 12, an arched roof 13 and opposite end walls14 and 15. The end wall 15 is provided with a charging slot 16 and endwall 14 has a similar discharge slot 17, the work W, shown as round barstock, being conveyed from the charging slot to the discharge slot bygravity, rolling upon spaced rails 18. In the ease of flat stock,suitable pusher mechanism may be employed to force the work along therails. Other forms of conveyance for the work may be provided, such as abelt or chain conveyor.

The charging and discharging slots 16 and 17 are provided with doors 20and 21, respectively, adapted to be opened and closed by conventionalmechanism, not shown.

An arched partition 22 extends transversely across the furnace, thelower end thereof being spaced above the floor a sufficient distance topermit passage of the work W therebeneath. The partition 22 serves topartially separate the furnace into two sections and aids in the supportof a pair of thin ceramic partitions 23 and 24, arched transverselyacross the furnace intermediate the floor 10 and roof 13, and thus withthe partition 22 divides the interior of the furnace into fourinter-connected chambers 25, 26, 27 and 28. Chamber 25 forms the low orpreheating zone of the furnace, communicating with chamber 26 whichserves as the final or high heat zone. Vents 29 extend from adjacent thefloor of the high heat zone 26, upwardly through the opposite side Walls11 and 12 (Fig. 3) and discharge into the upper chamber 27 through ports35. A group of vents 3l (Figs. l and 3) extend upwardly through the endwall 14 above the discharge slot 17 and also discharge into chamber 27through ports 32. A third group of vents 33 extend upwardly through theend wall above the charging slot 16, these vents communicating with ahorizontal passageway 34 which continues around in the side walls 11 and12 and discharges into chamber 27 through ports 35 (Fig. 2). Air jetnozzles 36, 37 and 38 are provided adjacent to the ports 32 and 35,respectively, in such manner as to induce a flow of gas from chamber 26through the vents 29 and 31 and from chamber 25 through the vent 33.

Passageways 40 extend through the upper portion of the arch 22,permitting relatively unrestricted passage of gases from chamber 27 tochamber 28, and chamber 28 is provided with a transverse vent slot 4l atthe forward end thereof adjacent to the end wall 15.

A series of burners 42 are disposed in the side Walls 11 and 12 beneaththe arched partition 24 and above the level of the work W. These burnersare directed substantially tangentially to the partition 24 so that theburner products will have scrubbing engagement with the lower face ofthis partition.

Burners 42 provide the entire fuel supply for creating the protectiveatmosphere in the heating chambers 25 and 26 and for the heatingrequirements of the furnace. The air-fuel mixture supplied to theburners 42, is one which on complete reaction will produce an atmospherecompletely non-scaling to steel. phere, as stated, will be one in whichthe sum of the CO2/CO and H2O/H2 ratios is approximately one. The actualCO2/CO and H2O/H2 ratios will vary with temperature each being about 0.5at l500 F. At 2100 F. the CO2/CO ratio will be about 0.3 and the H2O/H2ratio about 0.7. The proper air fuel mixture for obtaining this ratio islargely independent of the operating temperature of the furnace, beingcontrolled primarily by the ratio of molecular carbon to hydrogen in thefuel. Gaseous fuels having a C/H2 ratio below 1.0 will be restricted tofrom 50% to 54% of the air required for complete combustion in order toachieve the desired nonscaling condition, whereas the light oils, whichhave a C/Hz ratio of between 1.0 and 2.0 will produce the desired C02/COand H2O/H2 ratio summation with an air deficiency of from 40% to 50%.These rich mixtures release as useful heat only about to 25% of theavailable B. t. u. content of the fuel and develop temperatures only ofthe general order of 2100 F. Under normal furnace combustion conditionthey are This form of atmosincapable of reacting to completion withoutsome thermal assistance and hence incapable of producing the desirednon-scaling ratios. It is for this reason that the burners 42 aredirected upwardly against the partition 24, which, as will appear, ismaintained at a temperature several hundred degrees above the unassistedreaction temperature of the required air-fuel mixture. A portion of thereaction products created in the nal heat zone 26 will be drawn into thezone by the vents 33 and thus maintain the protective atmosphere thereinand under normal conditions of operation the amount of air supplied tothe induction nozzles 38 associated with vents 33 will be onlysuflicient to maintain the chamber 25 full against air infiltrationabout the door 20. The great majority of this rich atmosphere, whichstill contains from 75% to 80% of the available B. t. u. content of thefuel, is vented directly into chamber 27 through the vents 29 and 31under the suctional effect of the air nozzles 36 and 37, respectively.These nozzles are so spaced from their respective ports that thesuctional effect produced by the air required to complete the combustionof the atmosphere gases will maintain the chambers 25 and 26 under aslight positive pressure. Chamber 27 thus forms second combustionchamber to which an air-fuel mixture is supplied by the ports 32 and 35.Temperatures of the order of 2600 F. to 2800" F. are thus attainable inchamber 27. In order that a substantial portion of this heat will betransferred to the lower work heating chamber 26, the arch partition 24is preferably composed of thin interlocking refractory tiles composed ofa material of good heat conductivity, such as silicon carbide. This hotarch thus serves to heat the work in the chamber 26 both by radiationand by transfer of heat to the incoming burner products as they sweepover the hot surface thereof. Work temperatures as high as 2500 F. arethus obtainable in chamber 26.

Following this secondary combustion in chamber 27 the products thereofare drawn into chamber 28 by the vent 41 and thus pass over the secondarch partition 23 so as to impart a further portion of their heat to theincoming work in the preheat zone. Since the work in this zone isrelatively cool there will be a high heat differential between chambers28 and 25 and hence a rapid heat transfer to the latter. Thus the gasesvented from slot 4 1 will be at a considerably reduced temperature,depending upon the desired length of the preheat zone. This decrease intemperature of the secondary combustion gases between ports and vent 41represents the gain in operating eicieney due to the provision of thepreheat zone. In furnace having a Work temperature of the order of 2500F. such a decrease in the exhaust gas temperature of 500 F. representsan increase in operating heating eticiency of about 100%.

In order to further utilize the heat of the exhaust gases from vent 41an air supply conduit 42, for providing pressurized air to the nozzles36, 37 and 38, is positioned directly above the slot 41 transversely ofthe furnace, whereby preheating of the air will occur. Air is suppliedcentrally to this conduit through supply pipe 43 and is conducted tonozzles 36, 37 and 38 by manifolds 44 and 45 extending along each sideof the furnace. Individual tubes 46 and 47 supply the nozzles 36, atopposite sides of the furnace, from manifolds 44 and 45, respeetively.Air for nozzles 37 are supplied by tube 48 u from a secondary manifold49 which is connected at its opposite ends to manifolds 44 and 45,through normally closed electric valves 51 and 52, respectively.By-passes 53 and 54 extending around valves 51 and 52 are provided withmanual valves 55 and 56, respectively, whereby a predetermined low airsupply is fed to the nozzles 37 at all times. Nozzle 38 at one side ofthe furnace is connected to the manifold 44 through a normally closedelectric valve 57 and tubing 58, with a normally open by-pass 59disposed around the valve 57. Nozzle 38 at the opposite side of thefurnace is similarly connected to manifold 4S through an electric valve61 and by-pass 62, by tubing 63. By virtue of this arrangement aminimuni supply of air is provided to nozzles 37, 38 and 38' whichmaintain a suction in the door slot vents 31 and 33, respectively,sufficient to draw into these vents any oxidizing products of combustionwhich may be produced in the door slots by seepage of air about thedoors 21 and 20. Vents 33, as previously stated, also serve to maintaina low circulation of the non-scaling atmosphere from chamber 26 intochamber 25. Electric valves 57 and 61 are in circuit with a dooroperated switch 64 adapted to be closed, when the charging door isopened, to energize these valves to open position, thereby to augmentthe normal air supply to nozzles 38 and 38 and thus increase the suctionin vents 33 to remove a larger volume of combustion products produced inthe door slot 16 in the open condition of door 20. This increasedsuction also prevents flare out of the combustible atmosphere gases andburning at the door opening. .Electric valves 51 and S2, associatedwithI nozzles 37 are energized in a like manner by a switch 65 uponopening of the discharge door 21 whereby to similarly increase thesuctional effect in the vents 31 during the open period of this door.

In Figs. 4 and 5 I have shown a modified furnace in which the heatradiating partition 23 of Fig. l is omitted. In this embodiment thecompletely combusted gases pass directly from the secondary combustionchamber 27 into the preheat chamber 2S. The arched roof 13 is corbeleddown to adjacent the upper level of the work to bring the vented gasesdown over .the incoming cold work and thus lincrease the rate of heattransfer thereto. These gases are vented near the charging door 20 by anelongated vent slot 41.

The exhaust gases entering chamber 25 through the openings in the archpartition 22 are, of course, scaling to steel at elevated temperature.However, no appreciable oxidation occurs up to about 1200c F.,particularly when the heating period is short. The length of the chamber25', therefore, should not be greater than that required to bring thework up to a temperature of this order in the time of traveltherethrough. However, temperatures somewhat above this non-scalingrange may be tolerated as the work approaches the bulkhead 22 sincethere will be a small ow of non-scaling gas from chamber 26' to chamber2S beneath this partition which serves to blanket the work in chamber 21adjacent thereto. It is desired to reduce this flow to a minimum,however, and for this purpose the bulkhead 22 is fabricated with a flatlower face 70 closely spaced above the work so as to restrict the freespace beneath the bulkhead to a small proportion of the effective areaof the vent ports 29. The effective area of these ports is, of course,the actual area times a factor dependent upon the suctional effect ofthe air jet nozzles 36 and 37. With proper design it is possible toobtain factors up to l0. It is preferred to design the vents 29 toobtain approximately 90% venting therethrough and 10% or less beneaththe wall 22.

The combustible gas entering chamber 25 directly from chamber 26contains a large percentage of carbonmonoxide and hydrogen which willburn on contact with the outerv air at the slot 41. To avoid thisexternal burning and to utilize the available heat therein, I provideair jets 71 tiring into the chamber 25' from both ,sides thereof.

A supplemental vent 72 has been provided in the roof 'of the furnaceadjacent to the secondary combustion chamber outlet 40'. This vent isnormally covered by an adjustable refractory 73 so that the vent may beparytially opened to by-pass a portion of the combustion products shouldthere be a tendency to heat the work in chamber 25 above the safenon-scaling temperature. A similar adjustable refractory 74 is providedfor the 6 vent 41' to assist in the control of the relative flow throughvents 41 andV 72.

It is obvious that if it should be necessary to stop the movement of thework through zone 25 for any appreciable period with a portion of theload still in this zone, such work might become undesirably heated toabove the safe non-scaling temperature. To avoid scaling under suchconditions vent 41' may be completely closed by the refractory 74 andvent 72 opened fully. Since the oxidizing combustion gases enteringchamber 25 through opening 40 will be considerably higher in temperaturethan the non-scaling gases entering beneath the wall 22', thenon-scaling gases will tend to remain beneath the oxidizing combustiongases and form a protective atrnosphere about the work. It will, ofcourse, be understood that during such idling periods the volume of fueland air supplied to the burners 42 will also be reduced in accordancewith normal practice.

In Figs. 6 and 7 a batch type furnace of the duplex type is shown. Thisfurnace comprises two work heating chambers 75 and 76, separated by apartition 77; two secondary combustion chambers 78 and 79 which openfreely into each other through an opening 80 in the partition wall 77and are separated from the work heating chambers by thin archedpartitions 81 and 82 composed of a good heat conducting refractory; anda series of reaction tubes 83 to 88 which extend across the furnacewithin the secondary combustion chambers between the side walls 89 and90. Each of the reaction tubes abuts against a burner block 91 at oneend thereof and communicates at the other end with a passageway 92 whichopens into the work heating chambers 75 and 76 through ports 93. Theburner blocks 91 are provided with suitable burners 94, alternate tubesbeing fired from opposite ends. Thus, tubes 84, 86 and 88 have theirburners and burner blocks arranged in side wall as shown in Fig. 7whereas tubes 83, 85 and 87 have their burners and burner blocksdisposed in side wall 92.

The air-fuel mixture supplied to burners 94 is one which on completereaction will produce a non-scaling atmosphere or if desired, acarburizing atmosphere, that is a mixture having about 50% or lessaeration. The supplemental heat required for completing the reactions isobtained by secondary combustion of the products of these reactions withadditional air in the chambers 78 and 79. The gaseous products formed intubes 83 to 88 are passed into the chambers 75 and 76 to provide theprotective or work treating atmosphere. Ports 95 adjacent to the hearthor floor 96 of the furnace open into vents 97 which communicate withchambers 78 and 79 through burner tunnels 98, air jet nozzles 99 beingpositioned to direct a flow of air into the tunnels 98 in such manner asto produce a suctional effect in the vents 97 soV as to pull the richcombustible atmosphere from the Work chambers and induce it, withsupplemental air of combustion, into the secondary combustion chambers,Vents 101 and 102 are provided in the roof 103 for chambers 78 and 79,respectively, and vents 104 and 105 for the Work chambers 75 and 76 areprovided in the end walls 106 land 107, respectively. Vents 101 and 104may be closed individually or together by a sliding refractory 108 andvents 102 and 105 are similarly closed by a refractory 109.

Access to the chambers 75 and 76 for insertion and removal of work isobtained through door openings 110 and 111, respectively, normallyclosed by doors 112 and 113, respectively.

In the normal operation of the furnace the chambers 75 and 76 willoperate alternately as preheat and iinal heat chambers. Thus, if it beassumed that at a particular time chamber 76 is operating as a preheatchamber and chamber 75 as a final heating chamber, the valve block 108will be positioned as shown to close both vents 101 and 104. At suchtime the burners associated with tubes 83 to 85 will be operating tosupply the desired air-fuel mixture to these tubes Whereas the burnersassociated with tubes 85 to 88 will be turned od. The reaction productsfrom tubes 83 to 85 will pass into chamber 75 to protect the worktherein and impart heat thereto. Since the normal reaction temperatureof the mixture employed in the tubes S3 to 85 is only of the order of2100 F. whereas the combustion temperature of the secondary combustionproduced outside of the tubes in chamber 78 is of the order of 2600" F.to 2800 F. the reaction products within the tubes will absorb a largeportion of this external heat, thus insuring the completion of thereactions within the tubes and effectively transmitting the transferredheat into the work heating chamber. The use of the high temperaturereaction tubes, as compared with burners firing directly into the workchamber, as shown in Figs. l to 5 is particularly desirable whenemploying oil fuel in order to completely combine the carbon content otthe fuel with the meager air supply employed, and thus prevent sootformation. The hot products in passage to the vent ports 95 pass overthe work in chamber 75 giving up a portion of its heat thereto. It isthen mixed with additional air by the nozzles 99 associated with thevent ports 97 of chamber 75 and is burned in chamber 78, thus supplyingthe high heat head referred to for heating the tubes 83 to 85. The archpartition also is highly heated and thus serves as a radiant heat sourcefor the work in chamber 75.

The combustion products in chamber 78, which ordinarily would be ventedto the outer atmosphere, are now used to prelieat the work in chamber76, by passing the same into chamber 79, and over the arch S2 from whichthe work in chamber 76 will be heated by radiation. When it is desiredto keep the work being preheated from contact with these combustiongases, they may be directly vented from the furnace by opening the vent102. However, up to temperatures of about 1200 F. the oxidizing effectof these gases is .negligible to the work and during, at least thisportion of the preheat I prefer to close vent 102 and thus to force thecombustion products into chamber 76 through the fiues 97 associated withthis chamber whereby they will pass directly in contact with the workfor greater heating efficiency. They are then vented from chamber 76 bythe flue 105. After the work has attained a temperature at whichoxidation would occur in this atmosphere fiue 105 may be closed and vent102 opened to continue its heating from the radiant arch only until thework in chamber 75 has reached its final temperature. At this time theburners associated with tubes 83 to 85 will be shut off, refractory 108adjusted to open vent 101, refractory 109 moved to close vents 102 and105, the burners 99 associated with tubes 85 to S8 turned on and the airsupply to nozzles 99 associated with fiues 97 of chamber 76 turned on.Chamber 76 then serves a final heat chamber to bring the work therein upto its final temperature. The hot work may now be removed from chamber75 and a cold load inserted. The refractory 108 is then adjusted toclose vent 101 and open vent 106 whereby the secondary combustionproducts will be passed through chamber 75 to aid in the preheatingthereof.

When high production is desired it will be understood that both workchambers may be operated at high temperature, that is with burners 94 ofboth sets of tubes in full operation and air supplied to the nozzles 99of the fines 97 of both chambers. Under this condition of operationfines 104 and 105 will both be closed and vents 101 and 102 will both beopen so that the secondary combustion products will vent directly fromchambers 78 and 79 without passing through chambers 75 or 76.

Obviously, numerous other modifications of the lapparatus in which theprocess herein described may be carried out will occur to those skilledin the art without departing from the essential features of theinvention. The present embodiments are therefore to be considered in allrespects as illustrative and not restrictive, the scope 8 of theinvention being indicated by the appended claims rather than by theforegoing description, and all changes which come within the meaning andrange of equivalency of the claims are intended to be embraced therein.

What is claimed is:

l. A furnace for the heating of metal comprising a work heating chamberhaving -a preheating section and a final heating section, means forsupplying to said final heating section the hot primary products ofcombustion of a mixture of fuel and air having a large deciency of airfor complete combustion, -a combustion chamber disposed in direct heattransfer relation to said final heating section, means for venting atleast a portion of said primary combustion products from said finalheating section into said combustion chamber, means for addingadditional air to said vented products whereby to produce secondarycombustion thereof in said combustion chamber and means for passing theproducts of said secondary combustion in heat transfer relation to saidpreheating section.

2. A furnace constructed in accordance with claim l in which saidcombustion chamber is separated from said final heating section by apartition composed of -a refractory having good heat conductingcharacteristics.

3. A furnace constructed in accordance with claim 1 having means forventing a portion of the primary combustion products through saidpreheating section.

4. A furnace constructed in accordance with claim 1 having venting meansfor said preheating section whereby a portion of the primary reactionproducts are drawn through said preheating section.

5. A furnace constructed in accordance with claim 1 in which saidpreheating section is provided with a partition composed of a materialhaving good heat conducting characteristics and means for passing saidsecondary combustion products in Contact with one side of said partitionexternally of said preheating section.

6. A furnace constructed in accordance with claim 3 having means forintroducing air into said preheating section for secondary combustion ofthe primary combustion products vented therethrough.

7. A furnace constructed in accordance with claim 1 in which saidsecondary combustion products are passed through said preheatingsection.

8. A furnace constructed in accordance with claim 1 in which said meansfor supplying primary reaction products to said final heating sectioncomprises burners directed into said final heating section.

9. A furnace for the heating of metal having a first work heatingchamber -and a second work heating chamber, a partition at leastpartially separating said first and second work heating chambers, athird chamber separated from said first work heating chamber by apartition composed of good heat conducting material, a fourth chamberseparated from said second work heating chamber by a partition composedof good heat conducting material, means for supplying primary combustionproducts to said first work heating chamber having a large deficiency ofair for complete combustion, venting means extending from said firstwork heating chamber to said third chamber for passage of said primaryproducts to said third chamber, means for supplying air to said ventedproducts for secondary combustion thereof in said third chamber, meansfor passing said secondary products of combustion from said third tosaid fourth chamber and means for venting said secondary products fromsaid fourth chamber.

l0. A furnace constructed in accordance with claim 9 in which said meansfor venting the fourth chamber includes said second work heatingchamber.

ll. A furnace constructed in accordance with claim 9 having means forsupplying primary combustion products to said second work heatingchamber having a large deciency of air for complete combustion, meansfor venting said primary products to said fourth chamber, means foradding supplemental air to said vented products for secondary combustionin said fourth chamber and means for venting said third chamber to theexternal atmosphere, whereby said rst and second work heating chambermay be operated alternately as preheating and nal heating chambers.

References Cited in the le of this patent UNITED STATES PATENTS 975,077Rockwell Nov. 8, 1910 10 Dreein Mar. 4, 1941 Holcraft Mar. 18, 1952 NessApr. 12, 1955 Rusciano Sept, 18, 1956 FOREIGN PATENTS Great Britain Oct.22, 1925

1. A FURNACE FOR THE HEATING OF METAL COMPRISING A WORK HEATING CHAMBERHAVING A PREHEATING SECTION AND A FINAL HEATING SECTION, MEANS FORSUPPLYING TO SAID FINAL HEATING SECTION THE HOT PRIMARY PRODUCTS OFCOMBUSTION OF A MIXTURE OF FUEL AND AIR HAVING A LARGE DEFICIENCY OF AIRFOR COMPLETE COMBUSTION, A COMBUTION CHAMBER DISPOSED IN DIRECT HEATTRANSFER RELATION TO SAID FINAL HEATING SECTION, MEANS FOR VENTING ATLEAST A PORTION OF SAID PRIMARY COMBUSTION PRODUCTS FROM SAID FINALHEATING SECTION INTO SAID COMBUSTION CHAMBER, MEANS FOR ADDING ADDITIONAL AIR TO SAID VENTED PRODUCTS WHEREBY TO PRODUCE SECONDARYCOMBUSTION THEREOF IN SAID COMBUSTION CHAM-BER AND MEANS FOR PASSING THEPRODUCTS OF SAID SECONDARY COMBUSTION IN HEAT TRANSFER RELATION TO SAIDPREHEATING SECTION.